A variety of commercial chemical and petrochemical processes involve the condensation reaction of an olefin or a mixture of olefins over an acid catalyst to form higher molecular weight products. This process is referred to herein as a polymerization process, and the products can be either low molecular weight oligomers or high molecular weight polymers. Oligomers are formed by the condensation of 2, 3 or 4 olefin molecules with each other, while polymers are formed by the condensation of 5 or more olefin molecules with each other. As used herein, the term "polymerization" is used to refer to a process for the formation of oligomers and/or polymers.
Low molecular weight olefins (such as propene, 2-methylpropene, 1-butene and 2-butene) can be converted by polymerization over a solid acid catalyst (such as a solid phosphoric acid catalyst) to a product which is comprised of oligomers and is of value as a high-octane gasoline blending stock and as a starting material for the production of chemical intermediates and end-products which include alcohols, detergents and plastics. Such a process is typically carried out over a fixed-bed of solid acid catalyst and at elevated temperatures and pressures in either a chamber reactor or a tubular reactor. A plurality of reactors is ordinarily used in the practice of such a process so that individual reactors can be taken out of service for catalyst replacement or other maintenance without shutting down the other reactors of the process unit. In addition, reaction conditions in the process unit may be optimized through the use of two or more reactors in series.
The acid catalyzed alkylation of aromatic compounds with olefins is a well-known reaction which is of commercial importance. For example, ethylbenzene, cumene and detergent alkylate are produced by the alkylation of benzene with ethylene, propylene and C.sub.10 to C.sub.18 olefins, respectively. Sulfuric acid, HF, phosphoric acid, aluminum chloride, and boron fluoride are conventional catalysts which are useful for this reaction. In addition, solid acids which have a comparable acid strength can also be utilized to catalyze this process, and such materials include amorphous and crystalline aluminosilicates, clays, ion-exchange resins, mixed oxides and supported acids such as solid phosphoric acid catalysts.
When a fixed-bed of solid acid catalyst is used to catalyze the polymerization of olefins, great care must be taken to avoid deactivation of the catalyst. For example, small amounts of basic impurities in the feedstock can undergo chemical reaction with the acid catalyst and cause its deactivation. In addition, the reaction conditions must be carefully controlled to prevent the deposition of extremely high molecular weight products on the catalyst surface which will deactivate the catalyst. An undesirable increase in the pressure drop across the catalyst bed can also occur simultaneously with catalyst deactivation. It is particularly difficult to rapidly take a polymerization reactor out of service without causing undesirable catalyst deactivation.
In the operation of a petroleum refinery, a major source of olefins for an olefin polymerization unit will typically be a fluidized catalytic cracking unit. Any upset of the catalytic cracking unit or any other process unit that provides these olefins will, ordinarily, result in a reduced supply of olefins for the polymerization unit and will require that one or more of the process unit reactors be taken out of service. In the case of a polymerization unit that is operated using a solid phosphoric acid catalyst in a plurality of fixed-bed reactors, it is very difficult to rapidly take the reactors out of service without causing significant catalyst deactivation. A conventional shutdown procedure for such a unit, when attempting to save catalyst activity in the reactors, has been to recycle propane in the unit and then take the individual reactors off-line. Each individual reactor is then depressurized, residual hydrocarbons removed under vacuum, and the reactor purged with an inert gas such as nitrogen. Finally, the reactors are "blinded" by inserting a metal plate at the site of each valve which controls the flow of hydrocarbons through the reactors. This metal plate acts as a plug to prevent any possible leakage of hydrocarbons through the valve. Although catalyst activity can survive this type of shutdown procedure, catalyst coking is frequently observed as a consequence of the shutdown. It is not uncommon for catalyst deactivation to take place in at least one reactor of a polymerization unit which contains about eight tubular reactors and, occasionally, catalyst deactivation can take place in all of the reactors when the unit is shut down in this manner. For a conventional polymerization unit, this type of shutdown procedure can take as long as 16 hours.
Canadian Patent No. 1,055,921 (Burton et al.) discloses that a bed of solid phosphoric acid catalyst which has been deactivated by the deposition on the catalyst particles of polymerized and carbonized hydrocarbonaceous materials can be reactivated in situ by: (a) inundating the deactivated catalyst at a temperature of 40 to 370.degree. C. and pressure of 11/3 to 100 atmospheres, absolute, in a reactivating liquid of a mixture of hydrocarbons which is substantially free from sulfur, which contains at least 5 weight percent aromatics and which boils within the range of 40 to 230.degree. C.; (b) withdrawing the reactivating liquid from the catalyst; and (c) repeating steps (a) and (b), above, at least one time. It is further disclosed that the deactivated catalyst can be from a polymerization unit for production of motor fuel from light olefinic gases and that catalytic reformate can be used as the reactivating liquid.